If a person's leukopenia is caused by a bacterial infection, the treatment is to give them antibiotics. If it is drug-induced, the treatment is to stop giving the person the toxic medications for a while to let his or her bone marrow recover and start producing more white blood cells.

Most of the information in this writeup was gleaned from http://www.merck.com/pubs/mmanual/section11/chapter135/135a.htm. The rest is based on work I did for the sciencedictionary at http://biotech.icmb.utexas.edu/